FIG. 1 of the attached drawings shows the layout of a coupler with two inputs E.sub.1, E.sub.2 and 16 outputs S.sub.1 to S.sub.16, of a type known from French patent application no. 2,680,418 filed in the name of the Applicant (see corresponding U.S. Pat. No. 5,297,233 issued on Mar. 22, 1994, entitled "Integrated Optics Achromatic Splitter And An MXN Coupler Incorporating Such A Splitter")(see in particular FIGS. 9 and 10 of this document). Such a coupler is made, for example, by ion exchange on a glass substrate through a mask defining the configuration of waveguides connecting the two inputs to the sixteen outputs. As described in the aforementioned French patent application, the coupler essentially comprises a divider D and two groups of waveguides, each connected to one of the two divider outputs. Divider D comprises an "X" junction of two identical rectilinear waveguides, single mode at the wavelength used. This junction is connected to inputs E.sub.1, E.sub.2 in such a way that the optical power received from one of the two inputs is divided equally between its two output branches. To do this, as described in the aforementioned French patent application, the guides of junction D are inclined at an angle of roughly 1.degree. on axis of symmetry Z of the junction which is parallel to the general direction of light propagation, with coupler inputs and outputs connected to the waveguide branches parallel to axis Z.
Note that the waveguide layout shown in FIG. 1 is very strongly distorted in a ratio of approximately 10, parallel to this axis of symmetry which amplifies the curves in the waveguides; this improves clarity of the figure. Thus, the 2.times.16 coupler shown as an example can be formed on a substrate with a transverse dimension from about 1 to 6 mm for a longitudinal dimension L of around 46 mm, measured between the starts of the rectilinear input and output waveguide branches; these inputs and outputs having a transverse distribution interval of 350 micrometers.
The two groups of coplanar single mode waveguides connected to the 2 outputs of divider D are symmetrical relative to axis Z of the divider, with each group being composed of waveguide sections connected in the manner of a tree and branches by junctions J.sub.ii in a Y form, between an input junction J.sub.11, J.sub.21 respectively, connected to one of the outputs of the divider, and the N branches being rectilinear and parallel to axis Z of the divider, defining the N outputs S.sub.1 to S.sub.8 on the one hand and S.sub.9 to S.sub.16 on the other hand, of the coupler. A 1 to N coupler with such a tree and branches waveguide structure is described in French patent no. 2,676,548 (corresponding to U.S. application Ser. No. 876,954 filed on May 17, 1991) filed by this applicant, this coupler being characterized by the lack of deflection points on the different waveguides outside of the junctions themselves, a layout which ameliorates the longitudinal compactness of the coupler.
Couplers such as the one in FIG. 1 are used primarily in telecommunications, especially in frequency bands centered around 1310 nm and 1530 nm, that is, 1260 nm-1360 nm and 1480 nm-1580 nm, respectively. In these bands, then, it is important that division of optical power between outputs S.sub.1 to S.sub.16 have as few chromaticity flaws as possible.
FIG. 3 shows attenuation of optical powers at various outputs S.sub.1 to S.sub.16 of the coupler in FIG. 1, compared to the optical power injected into one of inputs E.sub.1, E.sub.2. These attenuations present themselves in particular in the first of two frequency bands mentioned above (that is, 1310.+-.50 nm and 1530.+-.50 nm) as strong spectral oscillations which are a function of wavelength, between 0.5 and 1 dB inclusive, which show an achromaticity capable of having a significant effect on the uniformity of optical powers at the coupler outputs. Likewise there is considerable "noise" at the outputs of the second frequency band, around 1530 nm. These phenomena persist even if a rectilinear waveguide section, for example 1 mm long, is positioned between divider D and the curved parts of the waveguides which connect it to input junctions J.sub.11 and J.sub.12.
This achromaticity flaw can be attributed to the fact that the mode profile at the output of an X junction is considerably deformed, more so than at the output of a simple Y junction, and that an X junction such as this one is less adiabatic.
The object of this invention is an integrated optical coupler with two inputs and 2N outputs which does not have a chromaticity flaw like the one in FIG. 1.